In the following explanations, any magnitude in a braking system by which the braking forces on the vehicle wheels can be influenced is understood to be a "braking value."
The "slip" of a wheel subjected to braking force is defined as the difference between the vehicle velocity and the rotational velocity of that wheel. The percentile vehicle velocity-share of the previously described slip is understood to be the "relative slip."
The locking of a wheel is understood to be a state in which the relative slip reaches a value of nearly 100%. At lower slip values, only a locking tendency exists.
A braking process of the kind described above is known from DE 44 10 937 C1. Braking processes of this type are generally used in combination with pressure-fluid actuated braking systems equipped with an anti-lock braking system (ABS). They can also be used in electronically controlled braking systems (EBS). To ascertain the rotational velocities of the vehicle wheels, sensors are used, and so-called pressure modulators are used to adjust the braking forces on the vehicle wheels. Vehicle wheels equipped with such sensors are called sensed wheels hereinafter. Solenoid valves, for example, are used as pressure modulators. The braking forces are determined on the basis of a signal which is hereinafter called a braking value. Any magnitude in the braking system which influences the braking forces can be used as a braking value. In a conventional braking system, the braking pressure influenced by the pressure modulator is normally used as the braking value. In an electronically controlled braking system, an electrical signal can also be used as a braking value.
When the vehicle is braked, locking of one or several wheels may occur, and this should be prevented by the anti-lock braking system (ABS), or by the electronic braking system (EBS). For this purpose, a control device evaluates the rotational velocity signals of the sensed wheels, and influences the braking forces at the wheels by means of the braking value.
For reasons of cost, each wheel on a vehicle axis is not equipped with a pressure modulator in certain vehicle models, e.g., in small trucks, but all the wheels of an axle are controlled with one single pressure modulator. When such a vehicle is braked on a road surface on which the frictional values (also called adhesion coefficients in the literature) are different on the two sides of the vehicle, the problem occurs that the wheels on both vehicle sides must be subjected to braking forces for optimal utilization of frictional values, but this is not possible on a vehicle axle with only one pressure modulator. On such a vehicle axle, the braking forces can only be adjusted together via one single braking value, for the determination of which a suitable regulating principle is necessary.
It is a known method to tie the rotational velocity signals of the wheels of that axle, or the regulating signals derived therefrom, into one single guiding magnitude, and to determine the braking value on the basis of this guiding magnitude. A process in general use consists of using the signals of the wheel located on the lower frictional value (low wheel) as the guiding magnitude (select low regulation). In this case, the maximum braking force of the wheel located on the surface with the higher frictional value (high wheel) transmissible to the road surface is not utilized, so that a relatively long braking distance results.
In the opposite case, it can also be seen that the signals of the wheel located on the surface with the higher frictional value may be used as guiding magnitude (select high regulation). In this manner, a relatively short braking distance is achieved, but with locking of the wheel located on the surface with the lower frictional value. This involves the danger that the tire may be damaged on the locking wheel, in particular if the lower frictional value increases again during the locking phase.
In the process for the braking of a vehicle mentioned initially, the proposal is made that when braking on a road surface with different frictional values on the two vehicle sides, the braking pressure be raised rapidly at first to a first value, and then be slowly raised to a pressure limit value, whereupon a select high regulation is carried out. Thanks to the slow increase of the braking pressure, a slow increase of the yawing moment or of steering forces is to be achieved. The select high regulation, by which the wheel located on the surface with the lower frictional value locks constantly, is maintained until full stop of the vehicle, or cessation of braking.
The above-described regulating concept has the disadvantage that as the originally low frictional value increases to a higher value during the select high regulation, the locking wheel often fails to start up again, so that the tire is damaged. Because the wheel locks constantly, it is also not possible to recognize such frictional value changes based on its rotational velocity signal.
It is therefore an object of the present invention to propose a process for the braking of a vehicle in which the braking forces applied to the wheels of an axle can be influenced by a common braking value at least at one axle, on road surfaces with changing frictional values, so that a short braking distance is achieved, and whereby frictional value changes can be detected at any time, in particular on a wheel which is located at first on a low frictional value.
It is a further object of the invention, that when braking on a road surface with different frictional values on the two sides of the vehicle, even a slight increase of the lower of the two frictional values is quickly detected. As a result, it is also possible to decide rapidly whether a continuation of the regulating principle used in that case, by which the wheel on the surface with the lower frictional value is locked at least intermittently, is acceptable, or whether a different regulating principle, by which the blocking of the wheel is avoided, must be applied henceforth because of expected tire damage.
Such a measure is based on recognition of the fact that a locked wheel utilizes only a relatively small portion of the available frictional value which results from the actual road surface condition. Since the locking of a wheel occurs when the braking force exerted by the braking system upon the wheel exceeds for a certain minimum time span the applicable frictional force, which is the utilized share of the available frictional value, the state of locking can be terminated correspondingly only when the utilized share of the frictional value in this state is able to provoke a frictional force which exceeds the braking force.
The above-mentioned condition can be met in principle when the locking wheel reaches a road surface segment on which the available frictional value is considerably greater, so that a sufficiently high frictional force is produced, in spite of the low utilization of frictional value during locking. In this case, the wheel starts up again without additional measures. No noticeable damage to the tire occurs.
In many instances however, the available frictional value will not suddenly increase to a considerably higher value, but will increase either slowly, or only slightly, so that the frictional force then produced by the utilized share of the frictional value is not yet sufficient to exceed the braking force. In this case, it may already be possible for the tire to become damaged.
Through the reduction of the braking forces at certain time intervals according to the invention, the conditions for a cessation of wheel locking can be met, however, during the time intervals, so that even in case of a slight increase of the available frictional value, the wheel is able to start up again after a short period of time, and can possibly be driven on with another regulating principle. In this manner, damage to the tires can be avoided.
It is another object of the invention to provide a second regulating principle in addition to the first regulating principle, where a wheel may lock for road surfaces with greatly different frictional values, said second regulating principle being preferably used on road surfaces with frictional values that are nearly equal on both sides, or which differ only slightly from each other, and where none of the wheels locks. Thereby, the lateral guiding force of the wheels is increased, and thus the travel stability of the vehicle during braking in a curve is improved. In this manner, an especially flexible type of brake force control can be achieved with many frictional value combinations occurring in practice.
It is still another object of the invention that the first regulating principle is not generally stopped when the lower frictional value increases, but that, depending on the magnitude of the increase which can be derived from the rotational velocity signals each time after a reduction of the braking forces according to the invention, a decision is made under which regulating principle travel is to be continued, while taking into consideration possible tire damage and the braking distance. As a result, an especially high level of safety is attained.
Yet another object of the invention is that the inventive process be used as part of a superimposed regulating concept in which different individual regulating principles are provided for different road surface conditions. An evaluation of the difference between the frictional values on the sides of the vehicle is provided for the selection of the required regulating principle in each case. For this, the rotational velocity signals are determined on both sides of the vehicle during braking by means of an integration over time, and from the difference of the time integrals, a ".mu.-split-value" is determined, which indicates the difference between the frictional values on the two sides of the vehicle. A suitable process for this is described in further detail in the prior art, e.g., in DE 44 10 937 C1, which is incorporated herein by reference.
In addition to the regulating principles already mentioned, an additional regulating principle may be provided which is used preferably on road surfaces with frictional values that are almost identical on both sides, or are only slightly different from each other, and where none of the wheels locks. The additional regulating principle may be identical with the second regulating principle.